Study On Ultrastructure And Physico-Chemical Properties In Pretreated Rice Straw

This study was proposed to investigate the effects of chemical treatments on ultrastructure and physico-chemical properties of rice straw to examine the probable mechanism with which chemical treatments exerted the effects on the improvement of nutritive value of straw.Rice straw was treated with sodium hydroxide (NaOH) at 15, 30, 45, 60 or 75 g/kg dry matter, ammonia bicarbonate (NH4HCO3) at 30, 60, 90, 120 or 150 g/kg or their combinations (10 or 20 g NaOH/kg and 30, 60 or 90 g NH₄HCO₃/kg), respectively. The efficiency of treatments was evaluated by the nutritive value of treated rice straw, the changes of physical properties were measured by some physical parameters, such as volumetric weight (VW), swelling capacity (SC), water-holding capacity (WHC), and specific porosity (SP), and histological changes of tissues and cell wall of rice straw after pretreatment with different chemicals and rumen degradation were observed by using a scanning electron microscope (SEM) and a transmission electron microscope (TEM).1. The efficiency of treatmentsThe matrix polysaccharides were decreased by treatment with NaOH, NH4HCO₃, or NaOH plus NH4HCO₃. The crude protein was increased by treatment with NH4HCO₃, or NaOH plus NH₄HCO₃. The dry matter degradability (DMD) of rice straws was improved by all the treatments in this experiment.2. The changes of physical propertiesThe analysis of orthogonal polynomial showed that NaOH treatment increased the VW and SC (P < 0.01) linearly, WHC quadratically (P < 0.05), and decreased SP (P < 0.01) of rice straw linearly. However, only WHC was decreased linearly (P < 0.01) with increasing NH4HCO₃. The SP was decreased and the SC was increased with increasing NH4HCO₃ (P < 0.05) in the combination treatment, while the interaction between NaOH and NH4HCO₃ affected these changes (P < 0.01).The SC and the WHC were directly affected by the concentration of cellulose or hemicellulose and crude protein or acid detergent lignin respectively, with correlation and regression-path analysis. There was a significant correlation between the DMD₂₄and VW (r = 0.703), SP (r = -0.767), SC (r = 0.766), or WHC (r = 0.474).3. Histological changes of tissues and cell wall of rice straw after pretreatment with different chemicalsEpidermis The papillas, wart-like structures, and micro-hairs on the cuticle layer, of stem epidermis were crimpled by treatment with NaOH at above 30 g/kg straw dry matter, but were not crimpled in NH4HCO3-treated stem. Apart of cuticle waxes silica layer was breached at high dosage of NH4HCO3. There were diffluence and breach in the combination treated epidermis, but these changes were less specific than these in NaOH or NH4HCO3-treated stem. The epidermis of NaOH-treated blade was distorted and the silica-cells were crimpled by treatment with NaOH.Tissues beneath the epidermis The parenchyma, vascular bundles and sclerenchyma tissues were crimpled by treatment with NaOH, which was a dose-dependent changes. The faultage between phloem of large vascular bundles and the parenchyma appeared at treatment with 60 or 75 g NaOH /kg dry matter. However, only parenchyma and large vascular tissues were slightly distorted in NH4HCO3-treated stem.Cells The cells of parenchyma and vascular bundles, and the cells of sclerenchyma located between sclerenchyma and parenchyma were more susceptible to chemical treatments than those sclerenchyma cells beneath the epidermis.4. Histological changes of tissues and cell wall of chemical treated rice straw after rumen incubationThe tissues and cells of epidermis The narrow-bands of epidermis were degraded at the same rate of the broad-bands. Obvious histological changes were observed in NaOH-treated stem at 12 h rumen incubation. Micro-hairs, papillas and wart-like structures did not exist, and only a membrane covered the underlying cells. When the stem was degraded for 48 h, only vertical ridge-like structure was left. However the epidermis was still observed in NH4HCO3-treated stems even after rumen incubation for 72 h, though the cracking begun at 12 h incubation, earlier than that of untreated stem, and the cuticle wax silica layer was thinner than that of untreated stem. The disappearance of cuticle wax layer and microbial degradation of underlying cell in NaOH-treated stem were observed more clearly under TEM than under SEM. It wasdifficult to realize which layer of epidermal cell was degraded under TEM, but it could be clearly observed that more bacteria had colonized into several layers of epidermal cell at 48 h than at 12 h rumen incubation.The tissues and cells beneath the epidermis It was difficult to say the degradation rate of NH4HCO3-treated stem was quicker than that of untreated stem. Obvious improvement of degradation could be seen in the NaOH-treated stems. ' After 24 h rumen incubation, not only the parenchyma and large vascular bundles were placed by sporangia of rumen fungi, but also the slight degradation of small vascular bundles could be observed under SEM. At 48 h, there were only a thin layer of sclerenchyma was left.The degradation of parenchyma cells and sclerenchyma cells located between parenchyma and sclerenchyma tissues was improved more easily by chemical treatments, in which the treatment effectivity of NaOH was higher than that of NaOH plus NH4HCO3, and than that of NH4HCO3. The degradation of vascular bundle cells was differed not only by chemical treatments, but also by the their location. The resistance of vascular bundle cells to bacteria attachment in the following order: the' periphery cells of pore < the periphery cells of metaxylem < the periphery cells of phloem < the cells between the two vessels of metaxylem.In summary, chemical treatments decrease cellulose and hemicellulose content of rice straw and result in swelling of cellulose molecules, decrease crude protein and lignin and result in increasing of water holding capacity of cellulose molecules. Swollen cellulose is easily penetrated by rumen fluid and this accounts for greater degradability of treated rice straw. Appeared at histological level, treatment with NaOH dissolves cuticle wax layer, and contracts parenchyma, sclerenchyma and vascular bundles, and hence increases the degradation of these tissue. While the effect of NH4HCO3 is less than that of NaOH, the cuticle wax layer may be breached â–  by treatment with NH4HCO3, but its degradation could not be improved. The degradation of NaOH-treated stem different to that of untreated or NH4HCO3-treated stem is bilateral, from inner and outer surface simultaneously.